Method and apparatus for adjusting the performance of a synchronous memory system

ABSTRACT

A method and apparatus for adjusting the performance of a memory system is provided. A memory system comprises a master device and a slave device. A memory channel couples the master device to the slave device such that the slave device receives the system operating information from the master device via the memory channel. The slave device further includes means for tuning circuitry within the slave device such that the performance of the memory system is improved.

The present invention relates to digital memory systems, and more specifically, to synchronous memory systems.

BACKGROUND OF THE INVENTION

As the operational frequencies of digital computing systems continue to increase, it has become increasingly necessary to use synchronous memory systems instead of the slower asynchronous memory systems. In synchronous memory systems, data is sent between a master device and one or more memory devices in the form of data packets which travel in parallel with, and must maintain precise timing relationships with, a system clock signal.

Because synchronous memory systems impose tight timing relationships between the clock and data signals, the memory interface circuits in the memory devices of the synchronous memory system generally require clock recovery and alignment circuits such as phase locked loops (PLLs) or delay locked loops (DLLs). One drawback of these clock recovery and alignment circuits, however, is that they typically operate effectively only over a limited range of frequencies. For example, a PLL may not be able to lock to the system's clock frequency if the frequency is either too low or too high. Additionally, the performance of these clock recovery and alignment circuits is degraded due to conditions such as temperature, supply voltage, speed binning codes, process, dimensions (i.e. length) of the memory bus, etc.

SUMMARY OF THE INVENTION

It is an object of this invention to provide for an adjustable synchronous memory system.

It is a further object of this invention to provide for a synchronous memory system that uses frequency information to improve the performance of the circuits at the system clock frequency.

It is a further object of this invention to provide for a synchronous memory system that uses system parameters to improve the performance of the circuits at the system clock frequency.

The present invention is a method for adjusting the performance of a synchronous memory system. A memory system comprises a master device and a slave device. A memory channel couples the master device to the slave device such that the slave device receives the system operating information from the master device via the memory channel. The slave device further includes means for tuning circuitry within the slave device such that the performance of the memory system is improved.

Other objects, features, and advantages of the present invention will be apparent from the accompanying drawings and from the detailed description that follows below.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings and in which like reference numerals refer to similar elements and in which:

FIG. 1 illustrates one embodiment of the synchronous memory system of the present invention.

FIG. 2 illustrates a portion of the synchronous memory system of FIG. 1.

FIG. 3 illustrates one embodiment of the memory interface circuitry inside a memory device of the present invention.

FIG. 4 illustrates a block diagram of one embodiment of a phase locked loop (PLL) circuit that may be used in the present invention.

FIG. 5 illustrates a block diagram of one embodiment of a delay locked loop (DLL) circuit that may be used in the present invention.

DETAILED DESCRIPTION

A method and apparatus for adjusting the performance of a memory system is described. A synchronous memory system wherein the master device has information about its operating frequency and transmits this frequency information to the memory devices is described. The memory devices then use this frequency information to adjust their clock recovery and alignment circuits to improve their performance at the system clock frequency. The master device may send the memory devices information that specifies the exact frequency of operation, or alternatively, the master device may send the memory devices information that specifies a predetermined range of frequencies which includes the system's clock frequency. For one embodiment, the frequency information is sent as a digital code that is received, stored, and decoded by the memory devices to produce a control code for adjusting the performance of the clock recovery and alignment circuits.

The synchronous memory system operates by sending and receiving data in packets which are synchronized with respect to a system clock. In order to do this properly, the memory master as well as all the slave devices must include circuitry that ensures that the data is read/written synchronously with the system clock. This circuitry is placed within the interface circuits of the memory master and slave devices. The key function of this circuitry is to produce internal clock signals within each device that maintain the proper phase relative to that of the external system clock such that data read or written to the channel by each of the devices is done so at the correct time, thereby preserving synchronization in the memory system. Because the memory devices may be used in different systems which use different operating clock frequencies, this circuitry should function effectively over a large range of possible system clock frequencies.

FIG. 1 illustrates one embodiment of the synchronous memory system of the present invention. This system comprises a master device 110, a memory bus 180, one or more memory devices 120, 130, a system clock source 150, and a terminator 140.

The master device 110 can be a memory controller, a microprocessor, a 3-D firmware chip, or any other microchip that accesses the synchronous memory. The master device 110 includes a memory interface circuit 115 for transmitting and receiving data from the memory bus 180. The master device 110 generates requests to store data into or recover data from the memory devices 120, 130 via the memory bus 180.

The memory bus 180 is a data communications channel. For one embodiment, the memory bus 180 is a collection of wires or transmission lines. For one embodiment, the memory bus 180 comprises matched-impedance printed circuit board traces.

For one embodiment, the memory devices 120, 130 are dynamic random access memories (DRAMs). Alternatively, the memory devices 120, 130 are static random access memories (SRAMs) or other memory devices. Each memory device 120, 130 includes a memory interface circuit 125, 135, respectively, for transmitting and receiving data from the memory bus 180. For one embodiment, the memory devices 120, 130 cannot generate requests for data but instead only respond to requests generated by the master device 110.

The clock source 150 provides the synchronizing clock signal for the memory system at a system clock frequency. In FIG. 1, the clock source 150 is shown providing this clock signal to a signal line called CTM 160 (clock-to-master). In this implementation, the clock signal travels along the memory bus 180 from the clock source 150, past all the memory devices 120, 130, to the master device 110. At the master device 110, CTM 160 connects to another signal line called CFM 170 (clock-from-master). Thus, the clock signal travels back along the memory bus 180 in the opposite direction, away from the master device 110, past all the memory devices 120, 130 and to the terminator 140. Data sent from the master device 110 to the memory devices 120, 130 travels in parallel with the clock signal on CFM 170. Likewise, data sent from the memory devices 120, 130 to the master device 110 travels in parallel with the clock signal of CTM 160.

The terminator 140 provides a matched-impedance termination for the transmission lines of the memory bus 180. For one embodiment, all signals transmitted on the memory bus 180 eventually terminate at the terminator 140. Although it is included in the synchronous memory system of FIG. 1, some synchronous memory systems do not use a terminator 140. For one embodiment, the terminator 140 is a plurality of resistors, coupled to the memory bus 180, preventing reflection of the signal.

FIG. 2 shows a more detailed drawing of a portion of a synchronous memory system. In order to show more detail, only the master device 110, one memory device 120, and the portion of the memory bus 180 that connects these two devices is shown. FIG. 2 shows the memory interface circuits 115, 125 for both the master device 110 and the memory device 120. Furthermore, the figure shows the clock recovery and alignment circuits 210, 220 (CRA circuits) within the interface circuits 115, 125, respectively. FIG. 2 also shows the internal clock lines 215, 225 which are driven by the CRA circuits 210, 220. These internal clock lines 215, 225 serve to synchronize the receive and transmit circuitry in the memory interface circuit 115, 125 to the system clock signals.

The master device 110 further includes information circuitry 290. The information circuitry holds information about the system's clock frequency and other system-level information. For one embodiment, the information circuitry 290 holds system clock frequency information. The information circuitry 290 may detect and/or store other information which affects circuit functioning. For one embodiment, the information circuitry 290 may detect and/or store information about the system temperature, or temperature ranges. The information circuitry 290 may detect and/or store information about the supply voltage, or voltage range. The information circuitry 290 may further detect and/or store information about the length of the memory bus 180, speed binning codes, process, and other factors that may affect the operation of the memory system. For one embodiment, the information circuitry 290 includes a PVTR detector. Detecting and storing this type of information is known in the art, as is the influence of the various factors on system operation. For one embodiment, the data in the information circuitry is also used to tune the performance of the CRA circuit 210 in the master device's memory interface 115.

For one embodiment, the same information about frequency, voltage, temperature, etc. that is sent to the memory devices to tune the performance of their CRA circuits is also made available to a CRA circuit inside the master device to tune its performance. Chip-specific information about the master device, such as the master device's process condition may be used along with the information that is sent to the memory devices to tune the performance master device's CRA circuit.

The master device 110 uses the memory bus 180 to access data and control the memory device 120. The master device 110 improves the performance of the CRA circuits in the memory device 120 by sending frequency information though the memory bus 180 to the memory device 120. For one embodiment, the frequency information is sent as a digital code to the memory device 120. Alternatively, for more accuracy, the frequency information may be sent as an analog signal.

FIG. 2 also shows the memory bus 180 in greater detail. The signal lines shown in FIG. 2 illustrate the types of signal lines present, in one embodiment. However, the number of signal lines illustrated do not correspond to the actual number of signal lines. The memory bus 180 includes the clock signal lines 250, CTM 160 and CFM 170, described above.

For one embodiment, the memory bus 180 includes a plurality of high-speed data lines 230, 270 which transmit data information between the master device 110 and the memory device 120 in parallel with either the CTM 160 or CFM 170 clock signals. The memory bus 180 also includes a plurality of high-speed control signal lines 240, 260 for transmitting address, request, acknowledge, and other control signals. Finally, the memory bus 180 includes lower-frequency “sideband” lines 280 for communicating information at lower speed between the master device 110 and the memory device 120.

For one embodiment, the memory channel includes slow speed lines and high speed lines. For one embodiment, the slow speed lines are used for system control such as nap, and the high speed lines are used for data and addressing. The controller is aware of its operating frequency and communicates this information to the slave devices. For one embodiment, the master device communicates this information to the slave devices via the slow speed lines. The slave devices receive, decode, and use this frequency information to adjust the circuits in their CRA circuits to improve their performance at the system clock frequency and other operating conditions. In other words, frequency control information comes down the slow speed lines to adjust/improve the performance of the high speed lines. For an alternative embodiment, there is only one channel that operates first at low speed to send frequency control information to adjust the CRA circuits and then operates at high speed after adjustment. For another alternative embodiment, there is only one channel that always operates at high speed, but until the frequency control information has been sent, it operates with lower initial margin.

For one embodiment, the master device 110 sends the memory device 120 information that specifies the exact frequency of operation. For another embodiment, the master device 110 sends the memory device 120 information that specifies a predetermined range of frequencies which includes the system's clock frequency.

For one embodiment, the master device further includes a PVTR detector, and the information circuit 290 further sends information from this PVTR circuit to the slave devices. In one embodiment, the master has a PVTR detector or other detector for detecting system operating parameters. The master sends this information to the slave devices so that they can adjust their performance. This data can be sent via any of the 3 ways described above, i.e. over a separate slow speed channel, over a temporarily slow speed channel, or over an initially low margin high speed channel.

In an alternative embodiment, each individual slave device has a PVTR detector to control the performance of its own CRA circuits.

In yet another embodiment, the master sends frequency information to the slaves, but each slave ALSO has its own PVTR detector. The frequency data is combined with the PVTR data to properly adjust the CRA circuits to account for both of these two types of operating information.

For one embodiment, such information is sent to the memory device 120 periodically during operation of the memory system. For another embodiment, the information is sent only once, during initialization of the memory system.

FIG. 3 shows a close-up view of one embodiment the memory interface circuitry 125 inside a memory device 120 of the present invention. The memory interface circuitry 125 includes a clock recovery and alignment circuit 310 (CRA circuit), transceiver circuitry 330, an n-bit wide register circuit 340, and an m-bit wide decoder circuitry 350. The transceiver circuitry 330 is designed to receive data from and/or transmit data to the memory bus 180. The m-bit wide decoder circuitry 350 is designed to decode the frequency information sent by the master device 110 and stored in the register circuit 340 to produce a corresponding m-bit control code.

The master device 110 (not shown) sends the information about the system's operating frequency to the memory device 120. The frequency information is encoded onto n bits as described below. This frequency information is then received by the transceiver circuitry 330 in each memory device 120. The use of transceiver circuitry 330 for receiving data from the memory bus 180 is well-known in the art. Upon receiving this information about the system's clock frequency, the transceiver circuitry 330 stores it into the n-bit register circuit 340. For one embodiment, the frequency information is stored in the register circuit 340 during normal system operation or at the initialization of the system.

The n-bit register circuit 340 presents this frequency information to a decoder circuitry 350. The decoder circuitry 350 translates this frequency information into a m-bit control code for adjusting the performance of the CRA circuitry 310. For one embodiment, the control code is used to adjust one or more portions of the CRA circuitry 310 such that the circuitry operates effectively at the system's clock frequency. For another embodiment, the control code adjusts the CRA circuitry 310 to optimize for external factors, such as temperature, memory bus 180 length, supply voltage, etc.

There are several ways that the frequency information can be encoded into n bits and then sent to and stored in the memory device 120. For one embodiment, a binary word indicates the time period of the system clock in pico-seconds (ps), where period=1 /frequency. For example, using a 16-bit register, a 16-bit digital word can be sent to the memory device 120 that indicates with 1 ps precision that the system's clock period is anywhere from 0 ps to 65,535 ps (2¹⁶−1). Alternatively, a more compact digital code that indicates one of a predetermined range of frequencies which includes the system's clock frequency may be used. This scheme requires the storage of fewer bits than the first scheme, but only specifies a range of frequencies instead of an exact frequency. One example of a compact code that could be used, and its corresponding range of frequencies, is shown in the table below:

TABLE 1 Code Indicating Frequency Range Minimum Maximum Frequency Compact Code Bits Frequency Frequency Range Size C1 C0 (fmin) (fmax) (Delta_F) 0 0 238 MHz 282 MHz 44 MHz 0 1 278 MHz 327 MHz 49 MHz 1 0 323 MHz 382 MHz 59 MHz 1 1 378 MHz 447 MHz 69 MHz

As can be seen, this scheme uses only 2 bits instead of 16 bits.

In one embodiment, a subset of the n-bit register is implemented. The number of bits used may be restricted to the minimum number needed for the applicable frequency range. For example, the full range of frequencies may use 16-bits, specifying a range of frequencies from 0 ps to 65,535 ps. One embodiment may implement 12-bits, limiting the range of frequencies from 0 ps to 4069 ps, if that is sufficient for the system application in question. For one embodiment, a similar method can be implemented for an encoded frequency information. Reducing the number of bits used reduces the circuitry needed to store and decoder the information, and thus reduces the cost of the device.

Once the frequency information has been stored in the n-bit register circuit 340, the decoder circuitry 350 evaluates this n-bit data to produce the required m-bit control code for adjusting the CRA circuitry 310 for optimal operation. For one embodiment, the control code that is decoded from the frequency information specifies a range of operating frequencies. This is simple if the register circuit 340 is given a compact code that specifies a range of frequencies which includes the system's clock frequency. The decoder circuitry 350 is more complex if the register circuit 340 holds the period of the system's clock signal. For one embodiment, the decoder circuitry 350 includes a simple look-up table for frequency ranges corresponding to control codes. For one embodiment, these tables are hard wired. For one embodiment, these tables may be altered by a user. The decoded control code is then sent to the CRA circuit 310.

The CRA circuit 310 adjusts the phase of internal clock signals so that the receive and transmit circuitry of the memory device 120 will be synchronized with the system clock signals CTM & CFM. The CRA circuit 310 may include variable delay elements, phase interpolator (mixer) circuits, and slew rate control circuits. By receiving and responding to these control codes, these circuits enable the synchronous memory system to operate effectively over a larger range of system clock frequencies than would be possible without the control codes. For one embodiment, the CRA circuit 310 is a phase locked loop (PLL) circuit. For another embodiment, the CRA circuit 310 is a delay-locked loop (DLL) circuit.

The control codes are used to adjust the locking frequency range of the clock recovery and alignment circuits to include the operating clock frequency of the system. The control codes are also used to reduce the jitter of the signals on the high-speed lines, and to improve the timing margin of the signals on the high-speed lines.

FIG. 4 illustrates a block diagram of one embodiment of a phase locked loop (PLL) circuit. The phase locked loop 400 is a feed back device that attempts to lock to the phase of an incoming signal. The phase detector compares the phase of the incoming signal 410 to that of the reference signal 460. The reference signal 460 is the output of the PLL 400, and it also serves as the feedback signal for the PLL system.

An input signal 410 is an input to the phase detector 420. For one embodiment, the input signal 410 is a system clock signal such as CTM. The output of phase detector 420 is an input to integrator/filter 430. The output of integrator/filter 430 is input to a voltage controlled oscillator (VCO) 440. The output of the VCO 440 is the output of the phase locked loop 400. The output of the VCO 440 is the reference signal 460, which is input to the phase detector 420.

An example of how the phase locked loop works is as follows. At the beginning the loop is in balance, i.e. the loop error is equal to zero. Assume that the frequency of the incoming signal increases slightly. This means that the phase of that signal changes a little faster—phase is the integral of frequency. Accordingly, the loop error becomes positive because the phase of the reference signal cannot change at once due to inherent delays in the PLL 400. The frequency generated by the VCO 440 follows the changes in the error signal so that it also increases. The final consequence is that an increase in the incoming signal's frequency causes an increase in the frequency of the reference signal. Thus, the reference signal 410 and incoming signal 410 converge on the same frequency. The elements of the PLL 440 are known in the art.

FIG. 5 illustrates a block diagram of one embodiment of a delay locked loop (DLL) circuit that can be used in the CRA circuit. An input signal is placed on input line 510 of the delay locked loop 500. The input signal is also an input to the delay element 550. The phase detector 520 functions to compare the phase difference between the input signal 510, and a feedback signal on feedback loop 570, and to generate two possible outputs, up, and down, representing the phase difference between the input signal and the feedback signal. The up and down signal outputs of the phase detector 520 are input to a charge pump 530. The charge pump is controlled by the up and down signals to raise or lower the voltage on its output line. The voltage on the output line is an input to a low pass filter 540, where it is filtered and delivered to delay element 550. The delay element 550 functions to delay the input signal, in proportion to the voltage delivered by the low pass filter 540. This delayed signal is the output signal on line 560, and is fed back to the phase detector 520 via the feedback line 570.

Although this disclosure has stressed the use of frequency information to tune the clock recovery and alignment circuit of the memory device 120, other relevant information could be transmitted to the memory device 120 and held by its register circuit 340 to tune the performance of clock recovery and alignment circuits. Examples of other types of information that could be sent from the master to the memory device 120 are temperature, supply voltage, speed binning codes, dimensions of the memory bus 180, etc. Any one or more of these types of information could be sent from the master device 110 to the memory device 120 to tune the memory device 120 circuits to operate more effectively under the system's operating conditions.

In the foregoing specification, the invention has been described with reference to specific exemplary embodiments. It will, however, be evident that various modifications and changes may be made without departing from the broader spirit and scope of the invention as set forth in the claims. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense. 

What is claimed is:
 1. A memory system comprising: an integrated circuit memory device including a clock synchronization circuit to synchronize transmission of data from the integrated circuit memory device with an external clock signal, wherein the clock synchronization circuit is turned based on operating information provided to the integrated circuit memory device; a master device to control the integrated circuit memory device; and a plurality of signal lines coupled to the master device and to the integrated circuit memory device, wherein the integrated circuit memory device receives the operating information from the master device via the plurality of signal lines.
 2. The memory system of claim 1 wherein the operating information specifies a clock frequency of the external clock signal.
 3. The memory system of claim 1 wherein the operating information specifies a frequency range from among a plurality of frequency ranges, the frequency range including a clock frequency of the external clock signal.
 4. The memory system of claim 1 wherein the operating information specifies a period of the external clock signal.
 5. The memory system of claim 1 wherein the master device further comprises a detector to determine the operating information, wherein the operating information specifies a temperature range.
 6. The memory system of claim 1 wherein the operating information specifies a supply voltage.
 7. The memory system of claim 1 wherein the operating information is representative of a length of at least one signal line of the plurality of signal lines.
 8. The memory system of claim 1 wherein the integrated circuit memory device includes a detector to determine an operating temperature range of the integrated circuit memory device, wherein the clock synchronization circuit is tuned based on the operating temperature range and the operating information.
 9. The memory system of claim 1, wherein the integrated circuit memory device includes a detector to determine a voltage range.
 10. The memory system of claim 9 wherein the clock synchronization circuit is turned based on the voltage range and the operating information.
 11. The memory system of claim 1 wherein the memory system is a synchronous memory system.
 12. The memory system of claim 1 wherein the clock synchronization circuit is a delay locked loop.
 13. The memory system of claim 1 wherein the clock synchronization circuit is a phase locked loop.
 14. The memory system of claim 1 wherein the clock synchronization circuit includes a mixer, wherein the clock synchronization curcuit is turned on using the mixer.
 15. The memory system of claim 14 wherein the clock synchronization circuit is turned to operate over a range of operating frequencies using the mixer, wherein an operating frequency within the range of operating frequencies is specified by the operating information.
 16. The memory system of claim 1 wherein the plurality of signal lines includes first lines which are operated at a first data transfer rate, and second lines which are operated at a second data transfer rate, wherein the second data trnasfer rate is lower than the first data transfer rate.
 17. The memory system of claim 16 wherein the integrated circuit memory device receives the operating information over the second lines, and wherein the data is transmitted over the first lines.
 18. The memory system of claim 16 wherein the integrated circuit memory device receives the operating information over the first lines.
 19. The memory system of claim 1 wherein the integrated circuit memory device further includes an n-bit register to store the system information.
 20. The memory system of claim 19, wherein the operating information is stored in a portion of the n-bit register.
 21. A method of controlling an integrated circuit memory device in a memory system that includes a master device, wherein the integrated circuit memory device is coupled to the master device via a plurality of signal lines, the method comprising: generating operating information in the master device; and sending the operating information from the master device via the plurality of signal lines to the integrated circuit memory device, wherein the integrated circuit memory device receives and stores the operating information, and wherein the integrated circuit memory device tunes a clock synchronization circuit using the operating information.
 22. A method of controlling an integrated circuit memory device according to claim 21 wherein the operating information specifies frequency information.
 23. A method of controlling an integrated circuit memory device according to claim 22 wherein the frequency information specifies an operating clock frequency of the memory system.
 24. A method for controlling an integrated circuit memory device according to claim 22 wherein the frequency information specifies a frequency range from among a plurality of frequency ranges, the specified frequency range including an operating clock frequency of the memory system.
 25. A method for controlling an integrated circuit memory device according to claim 22 wherein the frequency information specifies a period of an operating clock freqiency.
 26. A method for controlling an integrated circuit memory device according to claim 21 wherein the operating information includes information specifying supply voltage.
 27. A method of controlling an integrated circuit memory device according to claim 21 wherein the operating information represents a temperature range.
 28. A method of controlling an integrated circuit memory device according to claim 21 wherein the master device includes a clock synchronization circuit to synchronize transmission of data from the master device to the integrated circuit memory device, wherein the method further includes tuning the clock synchronization circuit within the master device.
 29. A method of controlling an integrated circuit memory device according to claim 21 wherein the operating information represents a length of at least one signal line of the plurality of signal lines.
 30. A method of controlling an integrated circuit memory device according to claim 21 wherein the memory system is a synchronous memory system.
 31. A method of controlling an integrated circuit memory device according to claim 21 wherein the master device includes a delay locked loop, the method further including tuning the delay locked loop using the operating information.
 32. A method of controlling an integrated circuit memory device according to claim 21 wherein the master device includes a phase locked loop, the method further including tuning the phase locked loop using the operating information.
 33. A method of controlling an integrated circuit memory device according to claim 32 wherein tuning the clock synchronization circuit of the integrated circuit memory device includes adjusting a locking frequency range of the clock synchronization circuit to include an operating clock frequency of the memory system.
 34. A method of controlling an integrated circuit memory device according to claim 21 wherein the plurality of signal lines includes a first plurality of lines and a second plurality of lines, wherein the second plurality of lines is designed to be operated at a slower data transfer rate than the first plurality of lines.
 35. A method of controlling an integrated circuit memory device according to claim 34 wherein tuning the clock synchronization circuit in the integrated circuit memory device reduces an amount of jitter of at least a first signal propagating on a first signal line of the first plurality of lines.
 36. A method of controlling an integrated circuit memory device according to claim 34 wherein the operating information is sent over the second plurality of lines.
 37. A method of controlling an integrated circuit memory device according to claim 34 wherein the operating information is sent over the first plurality of lines.
 38. A method of controlling an integrated circuit memory device according to claim 21 wherein the master device stores the operating information in an n-bit register.
 39. A method of controlling an integrated circuit memory device according to claim 38 wherein the operating information is stored in a subset of the n-bit register.
 40. The method of claim 21 wherein the generating and sending of operating information is performed periodically.
 41. The method of claim 21 wherein the operating information is sent during initialization of the memory system.
 42. A memory device having an array of memory cells, the memory device comprising: a programmable register to store a value that specifies a data transfer rate; and an interface circuit to output data to an external signal line at the data transfer rate specified by the value stored in the register.
 43. The memory device of claim 42 wherein the value is a binary code that is representative of one of a plurality of data transfer rates.
 44. The memory device of claim 42 further comprising a circuit to store temperature information.
 45. The memory device of claim 42 further comprising a clock synchronization circuit to receive an external clock signal and synchronize the outputting of data with the external clock signal.
 46. The memory device of claim 45 wherein the clock synchronization circuit comprises one of a delay locked loop circuit and a phase locked loop circuit.
 47. The memory device of claim 45 wherein the programmable register is coupled to the clock synchronization circuit and wherein the clock synchronization circuit is adjusted in accordance with the value stored within the programmable register.
 48. The memory device of claim 47 wherein the clock synchronization circuit comprises a mixer circuit to vary a phase of an internal clock signal in response to the value stored in the programmable register.
 49. The memory device of claim 42 further comprising decoder circuitry coupled to the programmable register to decode the value stored within the programmable register.
 50. The memory device of claim 42 further comprising a circuit to store information indicative of a length of an external bus.
 51. The memory device of claim 42 wherein the data is output synchronously with respect to an external clock signal.
 52. A memory device having an array of memory cells, the memory device comprising: a register to store a code that specifies a range of data transfer rates; and an interface circuit to output data to an external signal line at a transfer rate in the range of data transfer rates.
 53. The memory device of claim 52 wherein the code is representative of one of a plurality of data transfer rate ranges.
 54. The memory device of claim 52 further comprising a circuit to store operating temperature information.
 55. The memory device of claim 52 further comprsises an internal clock synchronization circuit to receive an external clock signal and synchronize the outputting of data with the external clock signal.
 56. The memory device of claim 55 wherein the internal clock synchronization circuit comprises one of a delay locked loop circuit and a phase locked loop circuit.
 57. The memory device of claim 55 wherein a frequency locking range of the clock synchronization circuit is varied in accordance with the code that specifies the range of data transfer rates.
 58. The memory device of claim 52 further comprising decoder circuitry coupled to the register to decode the code stored within the register.
 59. The memory device of claim 52 further comprising a circuit to store supply voltage information.
 60. A method of controlling a synchronous memory device, the memory device including a plurality of memory cells and a clock synchronization circuit, the method comprising: issuing information indicative of a data transfer rate to the memory device, wherein the memory device receives the information and adjusts the clock synchronization circuit using the information indicative of the data transfer rate; and transmitting data to the memory device at a transfer rate indicates by the information issued to the memory device.
 61. The method of claim 60 wherein the memory device stores the information indicative of the data transfer rate in a register on the memory device.
 62. The method of claim 60 wherein the information indicative of the data transfer rate is issued to the memory device during initialization of the memory device.
 63. The method of claim 60 further comprising: generating temperature information in a controller device; and providing the temperature information to the memory device, wherein the memory device adjusts the clock synchronization circuit using the temperature information.
 64. The method of claim 60 wherein the information indicative of the data transfer rate is a binary code that is representative of one of a plurality of data transfer rates.
 65. The method of claim 60 wherein the information indicative of the data transfer rate is a binary code that specifies a range of data transfer rates.
 66. The method of claim 60 wherein the memory device is coupled to a bus having a bus length, the method further comprising: generating bus length information based on the bus length; and providing the bus length information to the memory device.
 67. The method of claim 60 further comprising: detecting supply voltage information; and providing supply voltage information to the memory device, wherein the memory device adjusts the clock synchronization circuit using the supply voltage information.
 68. The method of claim 60 further comprising detecting the system operating frequency information and generating the information indicative of a data transfer rate based on the system operating frequency information.
 69. The method of claim 60 wherein the information indicative of a data transfer rate is issued to the memory device using an analog signal.
 70. A method of operation in a synchronous memory device, the memory device including a plurality of memory cells, the method comprising: receiving, from a controller, a value specifying a frequency; storing the value specifying the frequency in a programmable register on the memory device; and outputting data at the frequency specified by the value stored in the programmable register.
 71. The method of claim 70 wherein the value is received by the memory during an initialization period.
 72. The method of claim 70 wherein the data is output synchronously with respect to an external clock signal.
 73. The method of claim 70 further comprising: decoding the value to generate an internal control signal; and adjusting an internal clock synchronization circuit in response to the internal control signal.
 74. The method of claim 70 wherein the value is a binary code that is representative of one of a plurality of data transfer rates.
 75. The method of claim 70 wherein the memory device further comprises a clock synchronization circuit, the method further comprising adjusting the clock synchronization circuit based on the value stored in the programmable register.
 76. The method of claim 75 wherein the clock synchronization circuit comprises at least one of a delay lock loop circuit and a phase lock loop circuit.
 77. The method of claim 75 wherein a locking frequency range of the clock synchronization circuitry is adjusted in accordance with the value stored in the programmable register.
 78. The method of claim 70 further comprising generating an internal clock signal having a controlled phase relationship with an external clock signal.
 79. The method of claim 78 further comprising: detecting a phase differential between the external clock signal and the internal clock signal; generating the internal clock signal using a plurality of delay elements; and varying the amount of delay in each delay element based on the phase differential, to adjust the phase relationship between the internal clock signal and the external clock signal.
 80. The method of claim 70 further comprising: receiving supply voltage information from a controller; storing the supply voltage information within the memory device; and adjusting an internal clock synchronization circuit based on the supply voltage information.
 81. The method of claim 70 further comprising: receiving temperature information from a controller; storing the temperature information within the memory device; and adjusting an internal clock synchronization circuit based on the temperature information. 